This invention relates, in general, to circuits detecting pulse time of arrival (TOA) for radio frequency pulses, and more specifically, to processors for detecting real time TOA in variable and dense signal environments.
Pulse TOA has typically been measured against some fixed amplitude threshold. This method causes a variation to be perceived in the TOA due to the relative amplitude of the incoming pulse. Refinement to eliminate such variations was accomplished by the circuit found in U.S. Pat. No. 4,507,795, issued Mar. 26, 1985 to Carl Wagner and assigned to the same assignee as the present invention. The '795 patent includes a fixed delay and a threshold comparison channel to determine the leading and trailing edge TOA of the pulse. The '795 patent, in its preferred embodiment, shifts the comparison portion of the split signal down 3 dB, or another desired value from the pulse peak amplitude, and determines the leading edge TOA from the cross-over point of the threshold (non-delayed) signal with respect to the delayed signal.
The '795 method requires that the rise time of the pulse be short compared to the length of the delay line selected. The range of the pulse-widths need to be greater than the delay line selected. It is not practical to select a delay line length that will allow accurate TOA measurements for a range of pulse-widths and rise times occurring simultaneously over several orders of magnitude. Classically, the problem has been resolved by using a wave-form recording technique to record the entire pulse. The actual TOA value is then extracted through computer processing at later times. Real time determination is not possible through such a technique.